3.1. Model Evaluation
 Present-case RAMS temperatures (Figure 6, top ) generally compare well with observations (coefficient of determination (R2) = 0.87, Figure 7), as they capture diurnal cycles and day to day trends in peak-values; the 10 day average observed value was 19.1°C, while the modeled value was 20.3°C. RAMS captured the: large-scale cooling trend over the first three days, warming trend over the next four days, and final three-day cooling trend. Largest discrepancies occurred on the warm days, with overestimations of both their maxima by 2.5°C and minima by 2.0°C.
Figure 6. Modeled (red) versus observed (blue) hourly averaged (over the 12 METAR stations in Figure 5 for 1–10 June 2002): (top) temperatures (°C) and (bottom) wind speeds (m s−1).
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 Present-case RAMS wind speeds (Figure 6, bottom) also compare well with observations, as they again generally capture diurnal cycles and daily peak values (R2 = 0.8, Figure 7); the 10 day average observed speed was 2.9 m s−1, while the modeled value was 3.1 m s−1. Discrepancies exist, however, during the three hottest nights, which had observed near-calm winds versus an RAMS minima of about 1 m s−1; such stable SBL overestimations of near-calm speeds are common in meso-met models, and are probably related to deficiencies in their PBL schemes [Baklanov et al., 2011].
 Past-case JJA daily maximum surface RAMS temperatures (Figure 8) also generally compare well with observations (R2 = 0.7, Figure 9), as they capture day to day trends in peak values, i.e., large-scale cooling over the first 13 days, followed by warming over the next 13. Larger discrepancies again occur on the warm days, with overestimation of maxima by 2.5°C and minima by 2.0°C; the average observed Tmax value was 26.5°C, while the modeled was 27.3°C.
3.2. Climate Change Results
 Run-1 (present) results for Domain-1 (Figure 10, top) shows: surface mesoscale temperatures over the SoCAB basin at 1200 LT coldest over the ocean (down to 13°C), warmest inland (up to 30°C), a cool coastal strip, and cool higher-elevations east of the Basin. Up-slope winds (up to 2 m s−1) have developed over the mountains, while sea breezes (up to 2.5 m s−1) are confined to a narrow coastal strip. A near-calm region is located between the sea breeze and upslope winds, consistent with other SoCAB modeling studies for such hours [Boucouvala et al., 2003].
Figure 10. Run-1 (present) summer-averaged 19-m Domain-1 RAMS temperatures (°C) and wind speeds (1 barb is 1 m s−1) at (top) 1200, (middle) 1400, and (bottom) 1600 LT; box represents Domain-2 area, large arrows represent average over-ocean background-flows, and dashed lines are key topographic heights.
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 By 1400 LT (Figure 10, middle), the cool mountain tops persist, but inland temperatures have warmed (up to 32°C) and the cool coastal-strip (now at 23°C) is somewhat eroded from its value 2-h before. The sea breeze is, however, now strengthened (now up to 3.5 m s−1), penetrated further inland, and joined with the upslope winds on the eastern slopes. Two hours later (Figure 10, bottom), the cool coastal-strip and mountain-tops both still persist, while inland temperatures have reached their maximum (up to 35°C). Concurrent sea-breeze and upslope-wind penetration and intensity have also peaked (both up to 5 m s−1). Run-2 (past) results are not shown, as they look superficially similar to those of Run-1. Significant differences do exist, however, but can best be seen in the following “difference” plots.
 The Domain-1, 1200 LT Run-1 (present) minus Run-2 (past) difference-plots (Figure 11, top) show expected large-scale warming patterns, i.e., land temperatures have increased faster over the 35-year period than have ocean values (4.0 versus 0.5°C). The SoCAB shows the smallest warming, even cooling in a narrow coastal strip. Speed increases (as these difference-vectors are in the same general direction as the speed vectors of Figure 10) over the 35-year period in the sea breeze and upslope flows are largest over the ocean and inland mountain tops (up to 0.6 m s−1), while they are disorganized and small (only up to 0.2 m s−1) over SoCAB coastal areas, whose changes are better resolved in the Domain-2 results below.
 At 1400 LT (Figure 11, middle), coastal cooling over the SoCAB is better defined, while its speed differences have slightly strengthened (up to 0.4 m s−1) over the 2-h. By 1600 LT (Figure 11, bottom), speed differences have started to decrease from their peaks 2-h before, but coastal-cooling has maximized. The isolated over-ocean cooling at all three times (in the small area in the northwest of the domain) is not part of this “reverse-reaction” coastal-cooling, but part of a large-scale cooling in the NCEP BCs; discussion of its cause is beyond the scope of the current paper, but are discussed by Lebassi .
 In summary, the coarse Domain-1 results captured many important aspects of the observed coastal cooling (e.g., strengthened sea breeze, which resulted from increased temperature gradients that resulted from a smaller large-scale warming over the ocean than over inland areas).
 Domain-2 difference-fields at 1200 LT (Figure 12, top), which show more details than those of Domain-1 (Figure 11) because of its finer resolution, again show that over the 35 year period, the ocean warmed less than inland areas. Peak warming-values are smaller than in Domain-1, as Domain-2 does not extend into the peak-warming inland-areas. The urban part of the coastal SoCAB, however, has only slightly warmed over the 35-year period (up to 0.5°C), as the increased sea-breeze induced coastal-cooling has partially countered its large-scale warming, even producing two small pockets of weak coastal-cooling (up to −0.5°C).
 Sea breeze accelerations over the 35-year period in offshore coastal areas at this hour are more organized, and have larger vector-differences (up to 1.25 m s−1), than in the Domain-1 results. Smaller increases over the period, however, have occurred over the central urban-area than over the more rural land-area north of the city (0.75 versus 2.25 m s−1) because the large urban surface roughness (z0) minimizes the increased sea breeze flow (that formed due to large-scale warming) over the period.
 By 1400 LT (Figure 12, middle), the 35-year acceleration of the over-ocean flow from the Pacific High in the Domain-2 is strengthened over the last 2-h (difference vectors now up to 1.75 m s−1), as has the acceleration of the sea-breeze flow over the urban center (now up to 1.25 m s−1). Coastal cooling over the 35-years at this time at its maximum (between −0.8 and −1.0°C) than 2-h ago and now extends over the entire coastal strip.
 Two hours later (Figure 12, bottom), the 35-year over-ocean onshore acceleration of the previous 2-h has become an offshore directed acceleration. The coastal-plane sea-breeze acceleration is now weak and disorganized, as is the flow in areas between it and the still-organized up-slope flow. Coastal cooling has weakened, but is at its maximum inland-penetration, filling the San Fernando Valley, but stopped at the Chino and Santa Ana hills and at the San Gabriel Mountains; inland large-scale warming values over the 35-years continue to reduce from those 2-h before.
 Two-tailed t-tests on the 35-year RAMS Domain-2 changes in temperature (Figure 13, top) at 1400 LT (as well as at the other two hours, not shown) indicate that the coastal-cooling and (over ocean and inland) large-scale warming changes were significant at the 99% level, with less significant (<90%) values only at the boundary between the two areas. Both speed-component changes were also generally significant at 99% (Figures 13 (middle) and 13 (bottom)), except over a small coastal ocean area in the southern part of the domain, where differences are small.
Figure 13. Domain-2 two tailed statistical significance plots at 16 LT for (top) temperature, (middle) east-west wind-component, and (bottom) north-south wind-component.
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 The 13 western-most observed SoCAB cooling sites in Figure 1 are within the RAMS Domain-2 coastal-cooling area (including six of the seven highest-significant sites), while the four eastern most cooling sites are not. Nine of the 11 observed warming sites are likewise in the RAMS warming region (including three of four highest-significant sites). The remaining two observed warming sites are along the coast, southwest of the Santa Ana Mountains (which are parallel to the coast, just north of 117.5°W longitude), i.e., southeast of the RAMS cooling area. The RAMS coastal cooling area thus extends too far southward along the coast, but not far enough eastward in the inland area between the Santa Ana and San Bernardino Mountains (at the northeastern corner of Domain 2). The first effect resulted because of the 4 km Domain 2 grid spacing could not resolve the true elevation of the Santa Ana Mountains. The modeled coastal cooling flow was thus not able to be blocked by their true elevation, and thus the modeled flow went north (and not south) of them.
 In summary, Domain-2 fine-scale surface 35-year summer daytime changes better resolved (as compared to the coarser Domain-1 results) many important aspects of the observed coastal-cooling. These include its location, magnitude, and strengthened sea breezes, which resulted from increased temperature-gradients from the lower over-water large-scale warming rates, as compared to those over inland areas.